3GPP Long Term Evolution (LTE) project is an evolution and innovation for current 3G technologies, which can substantially increase spectral efficiency and cell capacity, and also can flexibly support multiple kinds of bandwidth configuration. The uplink of the LTE system adopts a single carrier frequency division multiple access (SC-FDMA) technology, which has low Peak-to-Average Power Ratio (PAPR) and can support a wide range of coverage effectively. Taking into consideration the constraint of the cost and the standardization progress of a UE, the existing uplink of the LTE system adopts a single-antenna rather than a multi-antenna technology to transmit signals of a UE. It should be noted that, the UE mentioned in the application can be a terminal equipment used by a user, such as a mobile phone.
The uplink of LTE has two kinds of reference signals: demodulation reference signal (DMRS) and sounding reference signal (SRS). The DMRS is mainly used for channel estimation for Physical Uplink Shared Channel (PUSCH), while the SRS is mainly used to detect quality of uplink channels. A Constant Modulus Zero Autocorrelation (CAZAC) code is adopted as a base sequence of the DMRS of the PUSCH in LTE, and more reference signals are obtained through different cyclic shift (CS) of the base sequence. The reference signal ru,v(α)(n) is generated by formula (1), wherein subscript u is the serial number of a group which the reference signal is separated into, and v is the serial number of the reference signal within the group.ru,v(α)(n)=ejαn ru,v(n), 0≦n<MscRS  (1)
In formula (1), ru,v(n) refers to the base sequence, MscRS refers to the length of the base sequence, and α refers to a cyclic shift value of the base sequence and is determined in accordance with formula (2).α=2πncs/12  (2)ncs=(nDMRS(1)+nDMRS(2)+nPRS) mod 12  (3)
In formula (3), nDMRS(1), nDMRS(2) and nPRS are three components for determining the cyclic shift value. Among them, nDMRS(1) is broadcasted in the cell; nDMRS(2) is designated by an uplink scheduling command; nPRS is generated based on a pseudo-random sequence, which is common in the same cell. In other words, with respect to the cyclic shift value, nDMRS(1) is a cell broadcasting component, nDMRS is an uplink scheduling component, and nPRS is a pseudo-random sequence component.
In order to reduce inter-cell interferences (ICI) between DMRSs of different cells, the cyclic shift value of the DMRS of each UE should hop at each slot in accordance with a predetermined pattern, similar to frequency hopping. In other words, the cyclic shift value of the base sequence is time-varying. Thus, a transmit antenna uses different reference signals as demodulation reference signals at different slots.
As the standard-setting work of LTE Rel-8 draws to a close, the research of LTE-A is started by 3GPP in the middle of 2008. LTE-A is a follow-up evolution of LTE, whose performance requirements are described in 3GPP TR 36.913. In comparison with LTE, a higher requirement is put on the system performance (in particular the performance of uplink) of LTE-A. For example, an LTE-A system requires that the peak spectrum efficiency of the uplink should reach 15 bps/Hz, which can not be satisfied by using only a single transmit antenna in the uplink of LTE. In order to meet the higher requirement on the uplink, it is considered to be necessary to use multi-antenna transmission technique in the uplink of LTE-A to further increase the spectrum efficiency.
Of course, a multi-antenna transmission system is much more complicated than a signal-antenna transmission system. There are many new issues to be considered, for example, how to provide demodulation reference signals for the uplink with multiple transmit antennas is an issue to be addressed.